1. Field of the Invention
Exemplary embodiments of the present invention relate to a frequency mixing apparatus; and, more particularly, to a frequency mixing apparatus applied to a frequency down-converter for a Ka-band satellite repeater, etc., which can effectively remove a second harmonic of a local oscillator from an output signal.
2. Description of Related Art
Recently, requirements of Ka-band communication satellites have been gradually increased so as to satisfy mobility and digitalization of communication rapidly spread. Signals used in Ka-band communication satellites have a radio frequency (RF) band of 30 GHz and an intermediate frequency (IF) band of 20 GHz.
Therefore, it is necessary to convert the frequency band of a signal from 30 GHz to 20 GHz. To this end, a local oscillator (LO) generating a signal with a frequency near 10 GHz is required. However, the signal generated from the LO passes through a frequency mixer, and the frequency mixer generates not only an IF signal of which frequency is converted but also an undesired second harmonic. The second harmonic has a value very close to 20 GHz.
Generally, a harmonic outputted from a mixer is removed using a filter, but it is very difficult to remove the harmonic very close to the IF band using the filter. Practically, a Ka-band communication repeater uses an RF band of 29.6 to 30 GHz and an IF band of 19.8 to 20.2 GHz.
Therefore, an LO signal of 9.8 GHz is required, and the mixer outputs not only an IF signal but also a second LO harmonic of 19.6 GHz which is just different by 200 MHz from the IF band.
The doubly balanced structure of a mixer is widely used as a general method for removing a second harmonic. However, although the mixer is implemented into the doubly balance structure, there is a limitation in removing the second harmonic.
In implementation of a practical frequency converter, several order filters are used together with a doubly balanced mixer. However, the several order filters increase the size of the frequency converter and also increase loss of an RF signal.
A representative technology for removing an LO harmonic has been disclosed in a paper published by Chin-Fu Li, Po-Chiun Huang, “A 60 dB harmonic mixing reduction mixer for wideband applications,” IEEE MTT-S International Proceedings, pp. 559-562, June 2008. In the paper, there is proposed a method for reducing a harmonic by adding two transistors to the structure of a Gilbert-type mixer and performing a rectification operation between the two transistors.
However, since a Gilbert-type analog circuit is used in the method, the circuit operates at only a low frequency of 1 GHz or less. Therefore, the circuit cannot be used at a high frequency. In addition, the two additional transistors increase the entire power consumption of the circuit.
U.S. Patent Publication No. US2006/0160518, entitled “Harmonic Reject Receiver Architecture and Mixer” has been disclosed in the art. In the patent, LO signals having different phases are applied to LO inputs of two mixers, respectively. Each of the LO signals having different phases is implemented by shifting an LO output using a phase shifter.
RF signals having the same phase are applied to RF inputs of the two mixers, respectively. Thus, IF (RF-LO) signals having different phases and LO harmonics having different phases are outputted as two IF outputs, respectively. Before the IF outputs, two mixers having the same type of the aforementioned two mixers are connected. The different RF signals become in-phase and the different IF signals become out-of-phase through phase shifters used for LO inputs of the added mixers, thereby removing the LO harmonics.
However, four mixers and two phase shifters are necessary for the aforementioned structure, and an LO is additionally provided. Therefore, the structure of the circuit is complicated, and the implementation cost of the circuit is increased.